The present invention relates generally to a ballast for gas discharge lamps. More particularly, this invention pertains to a ballast that may be used with both Metal Halide (MH) Lamps and High Pressure Sodium (HPS) Lamps.
Ballast devices for gas discharge lamps are well known in the art. For example, U.S. Pat. Nos. 3,599,037, 3,772,565, 3,873,910, 4,016,452, 4,162,428, 4,350,934, 4,501,994 disclose ballast devices that may be used to control various types of gas discharge lamps. None of these patents, however, disclose or suggest a ballast that may be used to control both a MH lamp and a HPS lamp.
Generally, a ballast designed to control a MH lamp (a MH ballast) may not be used to control an HPS lamp. An HPS lamp requires a starting aid, a low starting voltage, and a large current to operate properly. A MH ballast, on the other hand, does not include a starting aid and cannot be used to control an HPS lamp because an MH ballast provides a voltage that is higher than that required for the HPS lamp, as well as, a current that is lower than that required for the HPS lamp.
In addition, the voltage drop across an HPS lamp exhibits a wide range over the lifetime of the lamp. Thus, an HPS ballast is also designed to vary the current delivered to an HPS lamp in response to the voltage drop of the HPS lamp. As a result, the power delivered to the HPS lamp remains within a desired range over the entire range of voltages exhibited by the HPS lamp. A MH ballast does not provide the required variation in current. Furthermore, even if a starting aid is incorporated into a MH ballast, the HPS lamp will operate grossly under-wattage and exhibit wide variations in operating wattage as the HPS lamp exhibits its customary swings in voltage.
For similar reasons, an HPS ballast may not be used to control a MH lamp. If a standard MH lamp is used with an HPS ballast, the MH lamp will operate in an over-wattage state.
As a result of the incompatibility between HPS ballasts and MH ballasts, end users of MH and HPS lamps are required to purchase both types of ballasts, that is MH ballasts and HPS ballasts. The costs associated with purchasing both types of ballasts are undesirably high and end users have indicated a desire for a single ballast capable of operating both a MH lamp and an HPS lamp.
What is needed, then, is a combination ballast that may be used to operate both MH and HPS lamps.